Al-Musta'in

al-Musta'in (836-17 October 866) was the Abbasid Caliph from 8 June 862 to 17 October 866, succeeding al-Muntasir and preceding al-Mu'tazz.

Biography
al-Musta'in was the grandson of Caliph al-Mu'tasim, and he was elected caliph by a council of Turkish military leaders after the death of al-Muntasir. Arab and other troops in Baghdad mutinied against al-Musta'in, and they broke open the prison and plundered the armory before being bloodily suppressed. al-Musta'in continued the war on the Christian Byzantines, and two corps commanders and 8,000 men were killed during a major defeat at Lalakaon in September 863. Baghdad's Arabs rioted in response to this defeat and others, and a rebellion under Yahya ibn Umar broke out in 864, as did a revolt in Homs; both were crushed. In 865, the Turks mutinied against him after he attacked a Turkish captain during an inter-Turkish dispute, and the Turkish soldiers freed al-Mu'tazz from confinement. al-Muwaffaq and an army of 50,000 Turks and 2,000 Berbers besieged Baghdad, and al-Musta'in was executed after the city fell.